In recent years, by virtue of utilizing its useful chemical characteristics, titanium oxide has come to be used for various purposes. For instance, because titanium oxide has appropriate bonding power to oxygen as well as acid resistance, it is used as an oxidation or reduction catalyst, or used as a cosmetic material or surface coating material for plastics by utilizing its ultraviolet shielding characteristics, or used as a reflection-preventing coating material by utilizing its high refraction index. Further by virtue of a combination of these effects mentioned above, titanium oxide is used as a functional hard coating material and the like.
As stated above, titanium oxide is used for a variety of uses and in every case it is required to perform many functions. For instance, where titanium oxide is used as a catalyst, it is required to have not only activity on the main reaction but also selectivity, mechanical strength, heat resistance and acid resistance or service durability, and where titanium oxide is used as a cosmetic material, it is required to have not only an ultraviolet shielding effect but also smoothness, texture and transparency. Furthermore, where titanium oxide is used as a coating material, it is required to have excellent film forming properties, adhesive properties, film hardness, mechanical strength and abrasion resistance in addition to transparency and high refraction index.
As explained above, titanium oxide particles are used for various purposes and are required to have specific characteristics in accordance with the purpose for which they are used. In every case, however, the titanium oxide particles used are preferably ultrafine. Where the titanium oxide particles used are required to have transparency, it is essential that they not only are of uniform shape and size but also they are highly dispersible when incorporated into disperse media. To prepare such titanium oxide particles as satisfying these characteristics, it is preferable to use highly dispersed colloidal titanium oxide (titanium oxide sol).
Heretofore known as a process for preparing such titanium oxide sol as referred to above is the process wherein metatitanic acid obtained by hydrolysis of an aqueous salt of titanium solution is neutralized, followed by washing [deflocculation method of washed gel], or the process wherein an aqueous salt of titanium solution is deionized with ion exchange resins or the like to obtain a sol.
However, such processes for preparing titanium oxide sol as illustrated above involve problems as mentioned below.
(1) Because particles dispersed in a sol are of spherical or pseudospherical agglomerates and are not uniform in particle diameter, the sol becomes opaque white and is poor in transparency.
(2) Depending on the conditions employed under which a sol is formed, it is possible to obtain a sol having particles small in particle diameter and excellent in transparency, but, in this case, large amounts of acid as a stabilizer are needed.
(3) The sol is stable only when it is in an acid region (less than pH 3), and hence has its use limited because it forms precipitates, or gels when it is in a neutral or alkaline region.
(4) When the sol is mixed with an organic solvent or substituted therewith, it becomes unstable and forms precipitates, and hence the sol thus treated is difficult to use as a hard coating material to be applied to the surface of plastics or the like.
(5) Because particles present in the sol assume spherical or pseudospherical configuration, when it is used as a coating, for example, by mixing it with a base resin, the titanium oxide particles are not present densely in the coating thus formed, leaving spaces among the particles, and the density of the particles cannot be enhanced, and hence the coating obtained is poor in characteristics and strength as coating.
The present inventors conducted extensive research with the view of solving such problems as mentioned above, eventually accomplishing the present invention on the basis of their finding that if titanium oxide sol is prepared by a specific process, there can be obtained a titanium oxide sol having dispersed in a disperse medium the titanium oxide particles having such a specific shape as could not be obtained by the prior art processes for the preparation of titanium oxide sol, and the titanium oxide particles obtained in the titanium oxide sol prepared according to this specific process have various excellent characteristics.